1. Field of the Invention
The present invention relates to a driving circuit of an ink jet head using a piezo-electric actuator and a driving method of an ink jet head and, in particular, the present invention relates to a driving circuit of an ink jet head performing a high quality color image recording by jetting ink droplets and a driving method of the ink jet head.
2. Description of Related Art
In general, a conventional image processing technology for performing highly precise image recording has been of the gray level recording system using the area gray level correction of the dither system. Recently, however, it is requested to print an image having photographic quality at high speed. In order to satisfy such request, it is preferable to improve the image quality by providing a number of nozzles in an image forming device to realize a high speed printing and by controlling the image forming device such that minute ink droplets are jetted from the nozzles. Further, the size of ink droplet must be variable. This control system is called as droplet diameter modulation system. Normally, the size of ink droplet jetted from the nozzle can be regulated by controlling a voltage applied to the piezo-electric actuator.
An example of the conventional image forming device using the droplet diameter modulation system is disclosed in JP H10-315451A. In the disclosed image forming device, it is proposed that a driving waveform generated by a waveform generator and amplified by a power amplifier is supplied to all of piezo-electric actuators and the ink jetting is ON/OFF controlled by an image data.
Describing the proposed conventional system in detail with reference to FIG. 3, a drive waveform generated by waveform generator 312 is amplified by power amplifier 311 having a low output impedance to obtain a power capable of driving piezo-electric actuators 321 and ink droplets are jetted by opening/closing transfer gates 322 by the image data.
Another example of the proposed conventional system using the droplet diameter modulation system is disclosed in JP H9-174883 A. As shown in FIG. 5, which shows the another example, power amplifier 522 is provided for each of piezo-electric actuators 523 and the ink jetting is ON/OFF controlled by interface circuits 521 each for determining supply of drive waveform generated by waveform generator 511 to individual power amplifiers 522.
Incidentally, in order to jet minute droplet from an ink jet head using the piezo-electric actuator, drive waveform therefor has to have a large potential difference within a short time, that is, a large slew rate, and the slew rate of at least 10 (V/μs) is required recently. The piezo-electric actuator is a capacitive load and, when it is constructed with a laminated ceramics, electrostatic capacitance of each piezo-electric actuator is in the order of 3000 (pF). Further, since, in order to perform a high speed printing, the ink jet head has to have about 300 nozzles, a total electrostatic capacitance becomes up to 0.9 (μF). Therefore, the low output impedance power amplifier is used.
In a case of a serial printer, an ink jet head is mounted on a carriage and reciprocated perpendicularly to a moving direction of a printing sheet. A substrate on which power amplifiers are mounted is connected to piezo-electric actuators, which are loads of the power amplifiers, by a flexible cable. In such case, length of the flexible cable becomes 50 (cm) or more. Since electric resistance of the cable and electrostatic capacitance of the piezo-electric actuators form a low-pass filter, waveform of a voltage applied to a terminal of a piezo-electric actuator (referred to as “terminal voltage”, hereinafter) becomes dull, even if drive waveform having high slew rate can be obtained by using a power amplifier having low output impedance as in the technique disclosed in JP H10-315451 A.
FIG. 4 shows an influence of the low-pass filter on the terminal voltage of the piezo-electric actuator. In FIG. 4, it is assumed that electrostatic capacitance of each piezo-electric actuator is 3000 (pF), the number of nozzles formed in the ink jet head is 300, that is, a total electrostatic capacitance as a load of the voltage amplifiers is 0.9 (μF), and electric resistance of the cable is 0.5 (Ω). As shown, actual terminal voltage 42 of the piezo-electric actuator becomes dull compared with ideal waveform 41 due to the influence of the low-pass filter. The influence of the low-pass filter on the terminal voltage may cause the jetting of ink droplets to be unstable.
On the other hand, in order to realize the high speed printing, a number of nozzles must be provided. However, the larger number of the nozzles are provided the larger total electrostatic capacitance results, so that the cut-off frequency of the low-pass filter is lowered and dullness of the waveform of the terminal voltage becomes more remarkable.
The cut-off frequency depends upon a product of electric resistance and electrostatic capacitance of the cable. However, since the number of nozzles to be driven simultaneously is changed time to time, there is a problem that the dullness of the waveform varies correspondingly.
Since, in the technique disclosed in JP H9-174883 A, a plurality of power amplifiers 522 are mounted on the carriage with one piezo-electric actuator 523 being provided for each of power amplifiers 522, they are under influence of the low-pass filter. However, if the number of nozzles is increased in order to realize a high speed printing, the number of the power amplifiers must be increased, resulting in not only the size of the construction of the printer but also the amount of heat generation are increased much.
In order to solve this increased heat generation problem, the technique disclosed in JP H9-174883 A requires a heat radiation fan on the carriage. Therefore, weight of the carriage becomes large. On the other hand, in the ink jet printer of the serial type, the carriage must be driven reciprocally, an increase of weight of the carriage may cause vibration during acceleration and deceleration of the reciprocating carriage, resulting in that the image quality is degraded.
In order to avoid the vibration problem, sudden acceleration and sudden deceleration of the carriage must be avoided. However, in order to make the acceleration and deceleration of the carriage slow, a moving distance of the reciprocating carriage becomes long, causing the size of the printer to be increased.